Liquid crystals are applied to display media in which the reversible movement of liquid crystal molecules is made use of, such as display devices typically of TN or STN mode. Besides the application to display media, the liquid crystals have been studied for applicability to optically anisotropic materials or elements, such as a retardation film, a polarizer, a polarizing prism, a luminance-improving film, a low pass filter, and various optical filters, taking advantage of their anisotropy in physical properties, such as refractive index, dielectric constant, magnetic susceptibility, elastic modulus, and thermal expansion coefficient, as well as their alignment properties.
Retardation R of an optically anisotropic material, which relates to its anisotropy contrast, is represented by formula: R=Δn·d (n is optical (refractive index) anisotropy, and d is thickness). Since R should be set at a specific value, an increase in Δn allows a decrease in d. Thickness reduction of an optically anisotropic film makes it easy to control the alignment of liquid crystal molecules during polymerization, which will lead to improvements in yield and efficiency of the optically anisotropic element production.
The optically anisotropic material is obtained by, for example, uniformly aligning the molecules of a liquid crystal compound having a polymerizable functional group or a polymerizable composition containing the liquid crystal compound into a liquid crystal phase and irradiating the compound or the composition being in the liquid crystal phase with energy rays, such as ultraviolet rays, to cause photopolymerization. It is required to fix the aligned state of the liquid crystal compound uniformly and semi-permanently.
When the polymerizable composition has a high liquid crystal phase transition temperature, photopolymerization induced by energy rays may unintentionally invite thermal polymerization, which disturbs the uniform alignment of the liquid crystal molecules, making it difficult to fix a desired state of alignment. In order to facilitate temperature control during cure, a polymerizable composition showing a liquid crystal phase at or near room temperature is demanded.
The polymer is obtained by polymerizing the polymerizable composition in the form of coating film applied to a substrate. If the composition contains a non-polymerizable compound, the resulting polymer film may have insufficient strength or contain residual stress-induced strain. Removing a non-polymerizable compound using, e.g., a solvent can result in a failure to retain film homogeneity and cause unevenness. To obtain a polymer film with a uniform thickness, it is therefore preferred to apply a polymerizable composition in the form of a solution in a solvent to a substrate. Hence, it is desirable for a liquid crystal compound or a polymerizable composition containing it to have good solubility in a solvent.
A polymerizable liquid crystal compound having a (meth)acrylic group as a polymerizable functional group exhibits high polymerization reactivity, and the resulting polymer has high transparency and has therefore been extensively studied for use as an optically anisotropic material.
Compounds having a (meth)acryloyloxy group as a (meth)acrylic polymerizable group include a monomer having a phenylene group and/or a cyclohexyl group disclosed in Patent document 1, a reactive liquid crystal compound having a cyano-substituted phenylene group disclosed in Patent document 2, a nematic liquid crystal composition containing a diacrylate compound disclosed in Patent document 3, a mixture of a liquid crystal compound having a phenylene group disclosed in Patent document 4, and a compound obtained by the reaction of a mesogenic diol compound with a (meth)acryloyloxy group disclosed in Patent document 5.
In general, formation of a thick film from a polymerizable composition containing a liquid crystal compound encounters with difficulty in controlling molecular alignment of the liquid crystal compound, which can cause problems, such as reduction in transmittance and coloration. On the other hand, a thin polymer film having satisfactory molecular alignment over the entire area thereof can be obtained, but formation of a thin film has difficulty in thickness control, readily resulting in non-uniform surface condition or crystallization. Furthermore, the liquid crystal state resulting from alignment control in film formation is instable and tends to be disturbed before curing by irradiation with, e.g., UV light. Conventional known polymerizable compositions have thus failed to provide a satisfactory polymer.    Patent document 1: JP 2-6927A    Patent document 2: JP 6-16616A    Patent document 3: JP 6-240260A    Patent document 4: JP 11-513360A    Patent document 5: JP 2005-521538A